small company (around 20 employees)... small server... access to shared storage and sync outlook tasks/calendars... and maybe do some light processing on it
What sort of specs are recommended for running a Windows Server (mostly for Microsoft Exchange).
my first recommendation would be to talk to your boss, chief information officer, or president (depending on company) because today doing what you are asking is no longer a trivial matter, especially regarding data security, legal requirements, lawsuits, etc. So you don't just want to build any old computer/server and have it wherever. Talk to whoever would be held accountable for such a problem should it arise,
that may then lead to a dedicated server room being made available, having locked door or badged access, where you can set up resources and have it functional and reliable... reusing a nice gaming rig is not professional.
basically... workstations (i.e. your thought of gaming rig) are desktop pc's which will be limited to ~ 64gb or ram and are expected to be used by one person at a time. A server can be the same hardware, but will be in a rack server form factor, hold greater than 64gb of ram, and allow for better networking capability, to allow reliable and better performance to run MS exchange mail server and provide a data share.
you may want to look into NAS - network attached storage, which will be separate disks and hardware to provide data storage which would be separate from an MS exchange server; having one server do everything increases likelihood of everything not working one morning when something fails (and it will) so you have to plan all that out and understand what you actually need vs want vs budget. For however small a company, you just don't throw a gaming rig together to allow the company to do business; if it needs to be "cheap" then you may want to rethink your business strategy. You can find small business servers from DellEMC or HPE, you would call them tell them what you need and they will advise you on the type of hardware to be used, and you can always get refurbished/reconditioned if you are looking for less expensive equipment.